Article Figures & Tables

Figures

The eyes of the sea scallop Placopecten magellanicus. (A) Eyes arrayed along the valve mantle margins of a live P. magellanicus. The scale bar represents 5 mm. (B) Cross-section of an eye from P. magellanicus. The sample was stained with Hoechst dye, staining cell nuclei blue, and alpha-tubulin, staining green. The pigment layer underneath the mirror appears red in the image and in vivo. (C) A labeled diagram corresponding to (B). The scale bar in (B) and (C) represents 100 μm.

Examples of longitudinal chromatic aberration in (A) a camera eye, like those of fish or cephalopods, in which there is a lens with a high refractive index, a retina at the back of the eye, and no image-forming mirror and (B) a scallop eye in which there is a lens with a low refractive index and an image-forming, concave spherical mirror overlying a pigment layer at the back of the eye. In a camera eye, longitudinal chromatic aberration (LCA) causes shorter (bluer) wavelengths to be focused closer to the lens than longer (redder) wavelengths. Assuming that the scallop lens, like all biological lenses, produces LCA, shorter wavelengths come into focus further away from the lens than longer wavelengths.

Results from the microspectrophotometric analysis of photoreceptors from (A) the proximal retina of the sea scallop Placopecten magellanicus (λmax=487 nm), (B) the distal retina of P. magellanicus (λmax=509 nm), (C) the proximal retina of the bay scallop Argopecten irradians (λmax=502 nm), and (D) the distal retina of A. irradians (λmax=526 nm). The graphs present raw data from MSP recordings (with the baseline stripped and values smoothed over 10 nm intervals by a moving average) for a single representative photoreceptor from the retina and species indicated above. The data is overlain with a best-fit curve derived from an A1 rhodopsin template. The values presented here do not necessarily match those seen in Table 1, which presents the mean λmax for each set of photoreceptors.

Histograms of the results from the microspectrophotometric (MSP) analysis of individual photoreceptors from the (A) proximal and (B) distal retinas of the sea scallop Placopecten magellanicus and the (C) proximal and (D) distal retinas of the bay scallop Argopecten irradians. The values included with each histogram refer to λmax, the wavelength of peak absorbance for each photo-pigment studied, and is presented as mean ± s.e.m. Statistical comparisons were made using a Student's t-test (two-tailed). Significant differences (α=0.01) were found across both rows, which compare homologous retinas between species, and down both columns, which compare different retinas from the same species.

The fraction of incident light absorbed in a focused state by photoreceptors from the proximal (solid blue line) and distal (solid green line) retinas, after accounting for self-screening, in Placopecten magellanicus at a relatively (A) low (0.0067) and (B) high (0.0335) kD and Argopecten irradians at a low (C) and high (D) kD. Also shown are the absorbance spectra for proximal (dashed blue line) and distal (dashed green line) visual pigments that were used to calculate absorption by the photoreceptors. In both cases, the absorbance curves have been normalized to 1. As can be seen here, the absorption of unfocused light by scallop retinas, prior to their absorption of focused light, causes a shift in their peak sensitivities. Self-screening also affects the absorption spectrum of the distal retina more than the proximal retina.

Horizontal radiance spectra for (A) the offshore habitat of the sea scallop Placopecten magellanicus, modeled at 90 m, and (B) the inshore habitat of the bay scallop Argopecten irradians, modeled at 0.8 m. As illustrated here, P. magellanicus tends to live in deep oceanic water that is relatively dim and has a narrow radiance peak in the blue part of the visual spectrum, while A. irradians inhabits brighter, greener water.

The sum of the number of photons absorbed by one proximal receptor (Nprox) and one distal receptor (Ndist) in the sea scallop Placopecten magellanicus, at a relatively (A) low (0.0067) and (B) high (0.0335) kD, and the bay scallop Argopecten irradians, again at a low (C) and high (D) kD. For P. magellanicus and A. irradians an offshore environment at 90 m and an inshore environment at 0.8 m were assumed, respectively. In all four graphs, the white arrow marks the λmax values that maximize NP + ND; the gray arrow marks the λmax values empirically determined by MSP for a particular species; and the black arrow marks the λmax values for the other species. The formula used to calculate quantum catch accounted for the self-screening within the scallop eye, but the λmax values displayed above are for proximal and distal receptors prior to self-screening.

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Photo credit: Bret Tobalske.
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